Prior art fatigue monitors and passive fatigue monitors may be classified into four general methods. Visual crack length methods are the most numerous. Examples include Crane (U.S. Pat. No. 4,107,980), Smith (U.S. Pat. No. 3,979,949), Crites (U.S. Pat. No. 3,786,679), Brull (U.S. Pat. No. 4,590,804), and de la Veaux (U.S. Pat. No. 5,237,875). Change in electrical resistance methods include: Duframe (U.S. Pat. No. 4,255,974), Bohm (U.S. Pat. No. 3,765,230) and Prabhakaran (U.S. Pat. No. 5,227,731). Acoustic monitoring is illustrated in Green (U.S. Pat. No. 3,774,443) and (U.S. Pat. No. 4,265,120). Magnetic monitoring, the fourth method, was described by Satoh (U.S. Pat. No. 5,022,275 and U.S. Pat. No. 5,105,667). As noted by de la Veaux, the Brull invention (U.S. Pat. No. 4,590,804) has limited applications since the mounting techniques are improper and it is not adaptable for remote monitoring. He, therefore, patented improvements in the form of ASTM test standards for adhesives that provide superior bonding and he indicates wire attachments that could be monitored to detect changes in conductivity.
There are certain limitations in the prior art. An examination of the de la Veaux and Brull patents is illustrative of these limitations. As an example, the Brull patent, in FIG. 3, describes the cycles to failure as a function of stress amplitude. At a stress amplitude of 13 ksi, the plot predicts failure within 160,000 cycles for coupon 20 or 340,000 for coupon 14. Specific results from test on 7075 aluminum beam are presented in the table (Col. 7, 57-63) describing the cycles for failure to be 163,600 for coupon 20 and 286,700 for coupon 14. He also claims that the structural member being monitored failed at 486,000 cycles. Therefore, the claim is that a failure in coupon 14 represents 33% of service life and a failure in coupon 20 represents approximately 60%.
Both inventions assume that the "fatigue fuse" is placed on a new structure. Otherwise, the fuse does not represent an accurate percentage of service life when they fail. For example, if a fuse was placed on the above structural member that had already undergone 243,000 cycles, then the coupon 14 indication would be 67% instead of 33%. The structure would also have failed long before any indication by coupon 20. This situation illustrates major limitations of the Tensiodyne fatigue fuses. These limitations include the characteristic that fatigue monitoring applications are suitable only for existing structures and that fuses have no means of correction for prior stress history.
The fuse must be made from the same composition material as the structure being monitored for it to perform accurately. If the structure is coated, the fuse must have the same coating. This makes selection and use difficult since extra coating material must be kept on hand and material specifications for the structure must be available.
An additional limitation is that the de la Veaux patent claims to provide indication of three percentages of service life (Col. 7, 65-68) with no continuous or intermittent levels. If true monitoring is necessary, three data points would be insufficient over a structural loading period of many years. Also, the 75% maximum service life indication may not be sufficient on a 25 or 50 year structure. The remaining 25% (6 to 12 years) would not be monitored since all of the fuses would have already "blown".
Stress amplitudes vary considerably from one direction to another on a structure's surface and the previous inventions do not take this into consideration. The inventors apparently assume that the user will know to align the fuse with the principal (maximum) stresses in the structure since no method is described for doing so. If the cracks in the fuse are not perpendicular to the principal stresses, their propagation rates do not reflect the true damage in the structure and the fuse does not accurately monitor fatigue life.
Finally, the mechanical behavior of the test beam that was used by Brull as the basis for determining total service life and, therefore, the percentages predicted by the fuse elements, do not correspond to industry-accepted fatigue criterion for 7075 aluminum. An unnotched beam would require over 30 ksi to fail within 500,000 cycles, not 13 ksi. If the stress amplitude was 13 ksi, it would require over one trillion cycles to fail which is beyond any conceivable service life. Therefore, it is obvious that the test beam must have been notched and the total service life is highly dependent on the configuration of the notch. For instance, a 0.25 mm root radius notch in the test beam would reduce the service life from one trillion cycles at 14 ksi to approximately one million (the actual range is 1 to 10 million). A more acute notch may have dropped the cycles to the claim of 486,000 but, it can be seen that the accuracy of the fuse is very specific to the geometry of the fuse structure. If a range of 0.5 to 10 million cycles to failure is assumed, depending on the presence and shape of a notch, the percentage of life predicted by coupon 20 varies from 33% to 1.6%. The limitation imposed by this range of variance means that the fuse may only be used when the geometry of the structure is known and its fatigue behavior at the point of attachment has been characterized. As a result, fatigue fuses cannot be put into general use as the costly analysis required of each structural member, precludes economic use.